This disclosure is related to imaging systems that use a lens system to project images, and more specifically, to monitoring focus errors of imaging systems.
An imaging system can be configured to use an optical lens system to project an image of an object onto an imaging surface. The lens system may include one or more lenses depending on the imaging requirements for the system. In many applications, the distances from the object to the lens system and from the lens system to the imaging surface are carefully selected so that the image is well focused onto the imaging surface to form a sharp image. An imaging system may use a focus module to control the focus of the lens system. A photolithographic imaging system in wafer processing is an example of such an imaging system. Any focus error may cause blurring of the image and can be undesirable. For example, a focus error can adversely reduce the resolution in image sensing devices or undesirably change the minimum feature size in photolithography.
An optical lens system may be characterized by a number of parameters, including its numerical aperture ("NA"), depth of focus ("DOF"), and minimum feature size ("W") at its imaging surface. Under illumination of radiation at a wavelength (.lambda.), the minimum feature size due to the limit of diffraction is ##EQU1##
The depth of focus of this lens system can be expressed as ##EQU2##
It is desirable to make the minimum feature W as small as possible while maintaining a robust image fidelity. In a photolithographic process, a line feature in a photomask for a positive photoresist is in its best focus when its imaged linewidth is as large as possible. The image of the same line feature in a photomask for a negative photoresist is in its best focus when the imaged linewidth is as small as possible. High imaging resolution is achieved by either increasing the numerical aperture NA or using a shorter wavelength .lambda.. However, a large numerical aperture NA or a shorter wavelength .lambda. reduces the depth of focus so that any focus error is likely to cause significant blurring of the image. This therefore degrades the imaging resolution. Hence, a lens system may be optimized for the specific requirements of an application by selecting proper minimum feature size W and the depth of focus DOF.
The focus errors of an imaging system may be caused by various contributors which include, among others, the relative spacing between the lens system and the imaging surface, the relative spacing between the object and the lens system, and the non-flatness of the imaging surface. For example, in the exposure process of one type of photolithographic imaging systems such as a scanner lens system, the relative scanning motion between the wafer and the lens imaging system from one exposure to another may cause a variation or shift in the focus.